1. Field of the Present Subject Matter
The present subject matter relates to a composite adsorbent material that can be used in adsorption cooling and dehumidification systems, and a method for preparing the same.
2. Description of Related Art
In recent years, global warming and energy shortages have become more and more serious as economies develop rapidly all over the world. Adsorption cooling systems powered by solar energy or waste heat have drawn increasing attention, as such systems need neither chlorofluorocarbons (CFCs) nor hydrochlorofluorocarbons (HCFCs) as the working fluid, and neither fossil fuel nor electricity to drive them (R. Z. Wang et al, “An energy efficient hybrid system of solar powered water heater and adsorption ice maker,” Solar Energy, 68 (1), 2000, 189-195; R. Z. Wang et al, “Adsorption refrigeration: green cooling driven by low grade thermal energy,” Chinese Science Bulletin, 50 (3), 2005, 193-204; X. Q. Zhai et al., “A review for absorption and adsorption solar cooling systems in china,” Renewable and Sustainable Energy Reviews, 13 (6-7), 2009, 1523-1531; Y. Hamamoto et al., “Study on adsorption refrigeration cycle utilizing activated carbon fibers, part 2: cycle performance evaluation,” International Journal of Refrigeration, 29 (2), 2006, 315-327).
The working principle of an adsorption cooling system is that a large amount of the composite adsorbents packed in the adsorber adsorbs adsorbate, such as water vapor, from an evacuated container (the evaporator). So, the water in the evaporator continuously evaporates at low pressure to produce cooling which cools the process air. At the same time, the heat produced due to the adsorption of composite adsorbent is removed by cooling water in the adsorber. When the adsorption finishes, the composite adsorbent is heated by hot water/oil to desorb water to the condenser and then returns to the evaporator. Thus, it completes the thermodynamic cycle for both the adsorption and desorption processes. The hot water/oil is heated up by solar energy or waste heat which is free energy from the environment. The two adsorption/desorption chambers of the adsorption cooling systems work alternatively in order to produce the cooling effect continuously (Wang et al., 2000, supra and Zhai et al., 2009, supra).
Today, however, traditional vapor compression systems still dominate in almost all applications, since adsorption cooling has disadvantages that need to be improved. The primary disadvantages are: 1) long adsorption/desorption time; 2) low coefficient of performance (COP), leading to increased energy consumption and cost; and 3) low specific cooling power (SCP), leading to a bulky system. To overcome these problems, the adsorbent-adsorbate pair is a core element in the adsorption cooling system design and one direction is to develop new composite materials as effective adsorbents (Y. Li et al., “Adsorption refrigeration: a survey of novel technologies,” Recent Patents on Engineering, 1 (1), 2007, 1-21). Greater adsorption capacity can give a higher coefficient of performance. Similarly, a higher adsorption rate allows greater specific cooling power. Therefore, enhancing the adsorption properties, i.e., adsorption capacity and adsorption rate, of the composite adsorbent can definitely increase the value of COP and SCP (Wang et al., 2000, supra).
Silica-gel, activated carbon and zeolite 13X are each common adsorbents used in adsorption cooling systems. Each has its own strengths and weaknesses in terms of adsorption capacity. Zeolite 13X has excellent adsorption capacity at low pressures, but it requires heating to over 100° C. to desorb the adsorbate (R. A. Shigeishi et al., “Solar energy storage using chemical potential changes associated with drying of zeolites,” Solar Energy, 23 (6), 1979, 489-495), and it cannot adsorb and desorb large quantities of adsorbate within a narrow humidity/pressure range (Wang et al., 2009, supra).
Silica-gel can adsorb average amounts of water vapor at any pressure because of its hydrophilic properties (H. Kakiuchi et al., “Novel zeolite adsorbents and their application for AHP and desiccant system,” Presented at the IEA-Annex 17 Meeting, 2004, Beijing).
Activated carbon has a large internal surface area (commonly in the range of 1000-1500 m2 g−1) because of its high porosity and high surface reactivity, providing a large capacity for adsorbing chemicals from liquids or gases (A. Swiatkowski, “Industrial carbon adsorbents,” Studies in Surface Science and Catalysis, 120 (1), 1999, 69-94). Moreover, activated carbon is able to adsorb large amounts of water vapor at pressures above 1600 Pa. However, its water adsorption capacity at low pressures is weak. For an adsorption cooling system, a desirable adsorbent should have an S-shape adsorption isotherm with huge adsorption capacity at pressures from 750 Pa to 1100 Pa (Kakiuchi et al., 2004, supra). Activated carbon has an S-shape isotherm at such pressure range, but its adsorption capacity is low (R. A. Shigeishi et al., “Solar energy storage using chemical potential changes associated with drying of zeolites,” Solar Energy, 23 (6), 1979, 489-495).
There is a study showing that the best concentration of sodium silicate solution is around 0.1 to 10 wt. % for 48 hours of impregnation time (H. Huang et al., “Development research on composite adsorbents applied in adsorption heat pump,” Applied Thermal Engineering, 30, 2010, 1193-1198). However, the study did not test the effect of impregnating CaCl2 into the pores of activated carbon. It only studied the performance between activated carbon and silica-gel.
Silica-gel and activated carbon were employed for a composite material for adsorbent in order to achieve good performance in adsorption capacity (Huang et al., 2010, supra). CaCl2 and expanded graphite were used as adsorbents for an adsorption ice maker on fishing boats (Wang et al., 2006, supra). A new generation cooling device employing CaCl2 and silica gel as composite adsorbents was developed (B. B. Saha et al., “A new generation cooling device employing CaCl2-in-silica gel-water system,” International Journal of Heat and Mass Transfer, 52, 2009, 516-524). In addition, zeolite 13X and CaCl2 were used as composite adsorbents in an adsorption cooling/heating system (J. Li et al., “Composite adsorptive thermal energy storage material composed of zeolite 13X and calcium chloride,” Material Review (Cailiao Daobao), 19 No. 8, 2005, 109-113). Most of the composite adsorbents were found to enhance the SCP and COP. However, the adsorption capacity, COP and SCP of the adsorbents are still quite low.
Currently, only silica-gel is commercially used in adsorption cooling and dehumidification systems, because silica-gel can adsorb an average amount of water vapor at any pressure level due to its hydrophilic properties.
Accordingly, there has been a desire for a better composite adsorbent material for use in cooling systems and dehumidification systems.